A diesel particulate filter (DPF) is known as a filter that collects particulate material (PM) of an exhaust gas. Regenerating methods of burning the PM for continuous use of the DPF utilize heating with a burner or electric heater, fuel supply to the DPF having an oxidation catalyst by injection control, or the like. In a conventional method, the PM is fully burned every time the PM is deposited by a preset amount. The PM deposited in the DPF includes an impurity mainly composed of CaSO4 that is a combination of Ca in engine oil and S contained in diesel oil. The impurity, so-called ash, is generated in a combustion chamber of an internal combustion engine or within the filter by reaction between Ca and SOx through burning of the PM during the filter regenerating. A particulate diameter of the ash is between 0.1 μm to several μm and less than an average diameter 30 μm of filter pores. As the burnable parts of the PM is burned during the filter regeneration process, the filter pores stuffed with the PM as shown FIGS. 2A and 2B are opened as shown in FIG. 2C. The ash, which is incombustible, is thereby removed by passing through the filter pores with exhaust gas.
The regenerating method involves fuel consumption of the internal combustion engine through fuel supply to the filter, fuel supply to the burner, or fuel supply for generating electric power of the electric heater. Combustion speed of the PM has a tendency of decreasing with decrease of PM deposit amount. Combustion efficiency of the PM relative to the fuel consumption of the engine therefore decreases with the decrease of PM deposit amount during the regenerating process. In a conventional method, since the substantially entire amount of the PM deposited in the filter is burned every time once the PM deposit amount exceeds the preset amount, combustion efficiency on regenerating the filter is relatively low. This causes undesired increase of the fuel consumption in the regenerating method.